Chromium-based catalysts are used in the commercial polymerization of small alpha-olefins such as ethylene and propylene. One such catalyst is prepared by depositing chromocene (bis(cyclopentadienyl) chromium (II)) on an inorganic metal oxide support, as disclosed in British Patent No. 1,253,063 to Karapinka. U.S. Pat. No. 4,015,059, issued Mar. 29, 1977 to Karol, describes the use of bis(indenyl)- and bis(flourenyl)-chromium (II) compounds supported on activated inorganic oxide supports as catalysts for the polymerization of ethylene.
U.S. Pat. No. 4,530,914, issued Jul. 23, 1985 to Ewen et al., discloses a catalyst system for the polymerization of alpha-olefins which comprises two or more metallocenes, each having different propagation and termination rate constants, and aluminoxane. The metallocenes are cyclopentadienyl derivatives of a transition metal of Group 4b, 5b, and 6b metals of the Periodic Table. They are described by the formulas (C.sub.5 R'.sub.m).sub.p R".sub.s (C.sub.5 R'.sub.m)MeQ.sub.3-p and R".sub.s (C.sub.5 R'.sub.m).sub.2 MeQ' where (C.sub.5 R'.sub.m) is a cyclopentadienyl or substituted cyclopentadienyl, each R' is hydrogen or a hydrocarbyl radical, R" is an alkylene radical, a dialkyl germanium or silicon or an alkyl phosphine or amine radical bridging two (C.sub.5 R'.sub.m) rings, Q is a hydrocarbon radical, Me is a Group 4b, 5b, or 6b metal, s is 0 or 1, p is 0, 1, or 2; when p=0, s=0; m is 4 when s is 1 and m is 5 when s is 0.
U.S. Pat. No. 4,939,217, issued Jul. 3, 1990 to Stricklen, also discloses a process for polymerizing olefins where the polymerization is conducted in the presence of hydrogen, and a catalyst system is used which contains aluminoxane and at least two metallocenes, each having different olefin polymerization termination rate constants. The metallocenes disclosed are similar to those described in aforementioned U.S. Pat. No. 4,530,914.
U.S. Pat. No. 4,975,403, issued Dec. 4, 1990 to Ewen, discloses a catalyst system for use in the polymerization of olefins. The catalyst system includes at least two different chiral, stereo-rigid metallocene catalysts of the formula R''(C.sub.5 (R').sub.4).sub.2 MeQ.sub.P (where Me is a Group 4b, 5b or 6b metal and (C.sub.5 (R').sub.4) is a cyclopentadienyl or substituted cyclopentadienyl ring) and an aluminum compound.
Canadian Patent Application No. 2,000,567, published Apr. 13, 1990, discloses a process for producing polyethylene using a composite catalyst made up of a solid catalyst component typified by a selected chromium compound, a modified aluminum compound typified by a trialkylaluminum, and an alkylaluminum alkoxide compound. The chromium compound may be chromium oxide, and the modified aluminum compound may be the reaction product of an organoaluminum compound and water.
European Patent Application Publication No. 416,784 by Dawkins, published Mar. 13, 1991, discloses an olefin polymerization catalyst obtainable by depositing on a dry inorganic oxide support a mononuclear chromium complex to produce a catalyst precursor and thereafter by thermally activating the catalyst precursor. The mononuclear chromium complex is representable by the general formula: EQU Z--CR--L.sub.x
wherein Z is a cyclopentadienyl ligand substituted with 4 or 5 hydrocarbon groups containing 1 to 3 carbon atoms;
X is an integer from 1 to 4, and L is either a four- or five-substituted cyclopentadienyl ligand, or (CO).sub.3 R in which R is H, methyl or ethyl.
The catalyst is used to polymerize olefins, particularly ethylene optionally with C.sub.3 -C.sub.8 alpha-olefins. The catalyst is said to produce polyolefins having a relatively high molecular weight and a broad molecular weight distribution.
European patent Application Publication No. 416,785 by Dawkins, published Mar. 13, 1991, also describes an olefin polymerization catalyst obtainable by depositing on a dry inorganic oxide at least one mononuclear chromium complex having the general formula: EQU Z--Cr--L
wherein Z is a cyclopentadienyl ligand substituted with 4 or 5 hydrocarbyl groups containing 1 to 3 carbon atoms; and L is one or more hydrocarbyl ligands which are sufficiently reactive to enable the complex to react with the inorganic oxide without thermal activation.
The catalyst can be used without thermal activation for polymerizing olefins, particularly ethylene optionally with C.sub.3 -C.sub.8 alpha-olefins. The catalyst is said to produce polyolefins having a broad molecular weight distribution.
U.S. Pat. No. 4,424,139, issued Jan. 3, 1984 to McDaniel et al., discloses a catalyst system containing (a) a catalyst comprising a bis-(cyclopentadienyl)-chromium (II) compound and a phosphate-containing support and (b) a cocatalyst selected from trihydrocarbyl borane compounds and aluminum compounds. These catalyst are said to be useful as olefin polymerization catalysts, and are said to be capable of giving narrow molecular weight distribution polymer.
A tetrameric chromium (II) compound useful as an alpha-olefin polymerization catalyst is disclosed in U.S. Pat. No. 4,806,513, issued Feb. 21, 1989 to McDaniel et al. The compound, octakis-(.mu.-trimethylsilylmethyl)tetrachromium, is not, however, cyclopentadienyl-containing.
U.S. Pat. No. 4,587,227, issued May 6, 1986 to Smith et al., discloses octakis-(.mu.-trimethylsilylmethyl)tetrachromium on an inorganic oxide as a catalyst to make linear low density polyethylene with specific short chain branching.